Cross-border attacks increased 500 percent in the past year, from about 60 to more than 300, according to the military.

Allen said the problem with safe havens has gotten worse in the past 10 years.

"That's a question we have to ask the Pakistanis, in the end," he said. "My mandate ends at the border and I'll deal with the Taliban and the Haqqanis as they come across."

The Haqqanis are the arm of the Taliban that U.S. forces encounter mostly in the east.

"Those are the ones we are going to try to prevent from coming across over the border from the safe havens and when they do, we'll seek to deal with them," Allen said.

Allen said the majority of explosives were "probably" coming from Pakistan. The number of improvised explosive devices coming across the border hit a record high this summer at more than 5,000, killing at least 63 troops, according to the military.

"I have ordered a review of relationship of ISAF [International Security Assistance Force] with Pakistan and the intent will be to try to create opportunities for us to collaborate and cooperate in controlling that border," Allen said.

But Allen said the United States was seeking a "constructive relationship" with Pakistan.

"There's much worse than a bad relationship with Pakistan, which is no relationship with Pakistan," he said. "There are complicated dimensions to the relationship between Pakistan and Afghanistan, and Pakistan and the United States, and Pakistan and the international community. It's a complicated and multilayered development."

Dealing with the threat from Pakistan is getting harder. Allen's popular predecessor, now-CIA Director David Petraeus, had 100,000 troops to work with. By this time next year, Allen will have 40 percent fewer forces. Still, Allen is optimistic.

"I happen to believe we can be successful in doing this and I intend to lead this force to be successful," he said.

Allen takes charge at a time when the United States is increasingly becoming impatient with America's longest war. Public support for drawing down U.S. forces in Afghanistan as soon as possible spiked to 56 percent from 48 percent immediately after the killing of Osama bin Laden, according to the Pew Research Center.

Despite bin Laden's killing, Allen said the United States still has a job to do in Afghanistan.

"We came to this region for two reasons: go after al Qaeda," he said, "and to make sure the Taliban don't unseat this government and don't return to power."

Despite 10 years of war, 1,777 U.S. service members killed and $557.1 billion already spent -- for a public whose biggest concern is the faltering economy and lack of jobs -- the war is still worth fighting, Allen said.

"It is, it is," he said. "On the 11th of September, we were attacked by people who had planned, organized and executed the attack on the United States."